Multicomponent synthesis of chromophores – The one-pot approach to functional π-systems

Multicomponent reactions, conducted in a domino, sequential or consecutive fashion, have not only considerably enhanced synthetic efficiency as one-pot methodology, but they have also become an enabling tool for interdisciplinary research. The highly diversity-oriented nature of the synthetic concept allows accessing huge structural and functional space. Already some decades ago this has been recognized for life sciences, in particular, lead finding and exploration in pharma and agricultural chemistry. The quest for novel functional materials has also opened the field for diversity-oriented syntheses of functional π-systems, i.e. dyes for photonic and electronic applications based on their electronic properties. This review summarizes recent developments in MCR syntheses of functional chromophores highlighting syntheses following either the framework forming scaffold approach by establishing connectivity between chromophores or the chromogenic chromophore approach by de novo formation of chromophore of interest. Both approaches warrant rapid access to molecular functional π-systems, i.e. chromophores, fluorophores, and electrophores for various applications.

Phenolphthalein Conjugated Schiff Base as a Dual Emissive Fluorogenic Probe for the Recognition Aluminum (III) and Zinc (II) Ions

In this study, a new phenolphthalein derivative, FFIZNA, has been planned and successfully prepared in an uncomplicated way. The probe FFIZNA could selectively monitor Al³⁺ and Zn²⁺ among other relevant cations with diverse colors through a turn-on emission response in EtOH:HEPES (9/1;v/v) media owing to the chelation enhanced fluorescence (CHEF), prevention of ESIPT, -C=N- isomerization and PET of the probe FFIZNA. The interactions of Al³⁺ and Zn²⁺ with the probe FFIZNA were confirmed by emission spectroscopy, Job's plot and ¹H-NMR titration substantiated 1:2 reaction stoichiometry between FFIZNA and Al³⁺ and Zn²⁺. The time-response study displayed that the emission of FFIZNA with Al³⁺ and Zn²⁺, rapidly boosted and reached the stable value in less than 3.0 and 4.0 min, respectively. Therefore, the FFIZNA has successfully been utilized to the dual recognition of Al³⁺ and Zn²⁺ in solutions. Phenolphthalein conjugated schiff base as a dual emissive fluorogenic probe for the detection aluminum (III) and zinc (II) ions.

A novel ratiometric AIE-based fluorescent probe for specific detection of Hcy/Cys and imaging of living cells in vivo

The sensing mechanism toward Hcy/Cys is realized based on the condensation reaction, which breaks CN to form a thiazolidine adduct.

Thermally responsive AIE-active polyurethanes based on a tetraaniline derivative

Polyurethanes with different soft–hard segment ratios were successfully synthesized, with an aggregation-induced-emission (AIE)-active tetraaniline derivative (NH₂–B₃–Ani₄–NH₂) as the hard segment. The resulting polyurethanes exhibited typical AIE features. The fluorescence intensities of polyurethane films changed with heat treatments. The fluorescence intensities of the polyurethane films decreased sharply after quenching treatment, yet their fluorescence intensities exceeded the original intensities of the films after thermal annealing at 80 °C for 24 h. Differential Scanning Calorimetry (DSC) results implied that the melting peaks in polyurethane films disappeared after quenching treatment, but the melting peaks appeared again after thermal annealing. These results proved that the arrangement of the structure had an important effect on the AIE properties of the polyurethane films. Meanwhile, the fluorescence intensities of these polyurethanes decreased with the increase of temperature, indicating that all three polyurethanes exhibited temperature-dependent fluorescent characteristics. Based on the above investigations, the AIE-active polyurethanes may provide a platform for the development of stimuli-responsive fluorescent materials.

Polyurethanes with an AIE fluorophore (tetraaniline derivative) are thermo-responsive, demonstrating that AIE-active polyurethane films have promising applications in stimuli-responsive materials.

Application of real sample analysis and biosensing: Synthesis of new naphthyl derived chemosensor for detection of Al3+ ions

A new chemosensor (NANH) based on naphthyl moiety was synthesized with good selectivity and sensitivity towards Al³⁺ ions via the inhibition by operating through dual mechanisms like photo-induced electron transfer (PET) and excited-state intramolecular proton transfer (ESIPT). The synthesized NANH was validated by various techniques such as ¹H, ¹³C NMR and mass spectrum. While prominent fluorescent enhancement was observed from the NANH upon binding with Al³⁺ ions, however, other metal ions have not responded in the emission spectrum. Detection limit and association constant of NANH for Al³⁺ were calculated as 1.2 × 10^-7 M and 4.09 × 10⁴ M^-1 by using fluorescence titration method. Binding ratio (1:1) of NANH with Al³⁺ ions were proved by Job's plot and DFT studies. Furthermore, aluminium in variety of water samples was determined, and NANH could be used for biosensing of Al³⁺ in living cells.

AIEgens: An emerging fluorescent sensing tool to aid food safety and quality control

As a global public health problem, food safety has attracted increasing concern. To minimize the risk exposure of food to harmful ingredients, food quality and safety inspection that covers the whole process of "from farm to fork" is much desired. Fluorescent sensing is a promising and powerful screening tool for sensing hazardous substances in food and thus plays a crucial role in promoting food safety assurance. However, traditional fluorphores generally suffer the problem of aggregation-caused quenching (ACQ) effect, which limit their application in food quality and safety inspection. In this regard, luminogens with aggregation-induced emission property (AIEgens) showed large potential in food analysis since AIEgens effectively surmount the ACQ effect with much better detection sensitivity, accuracy, and robustness. In this contribution, we review the latest developments of food safety monitoring by AIEgens, which will focus on the molecular design of AIEgens and their sensing principles. Several examples of AIE-based sensing applications for screening food contaminations are highlighted, and future perspectives and challenges in this emerging field are tentatively elaborated. We hope this review can motivate new research ideas and interest to aid food safety and quality control, and facilitate more collaborative endeavors to advance the state-of-the-art sensing developments and reduce actual translational gap between laboratory research and industrial production.

Detection of Al3+ and Fe3+ ions by nitrobenzoxadiazole bearing pyridine-2,6-dicarboxamide based chemosensors: effect of solvents on detection

Hybrid chemosensors, based on a pyridine-2,6-dicarboxamide fragment while containing a 4-nitrobenzoxadiazole group, are used for the sensing of Al³⁺ and Fe³⁺ ions where detection was significantly controlled by the selection of a solvent.

A sensitive OFF–ON–OFF fluorescent probe for the cascade sensing of Al3+ and F− ions in aqueous media and living cells

A simple Schiff-base ligand 2-hydroxy-1-naphthaldehyde semicarbazone (HNS) was synthesized and characterized. Based on the combined effect of inhibition of CH Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N isomerization and chelation-enhanced fluorescence (CHEF), HNS functions as a fluorescence “turn on” sensor for Al³⁺ in buffered aqueous media. Based on the strong affinity of Al³⁺ to F⁻ ions, the in situ generated Al³⁺–HNS complex can also be utilized as an effective chemosensor for F⁻ sensing by metal displacement approach, ensuing quenching of fluorescence by the reversible return of HNS from Al³⁺–HNS complex. Thus a method using a single probe for the detection of both Al³⁺ and F⁻ ions is developed. The system exhibits high selectivity and sensitivity for Al³⁺ and F⁻ ions and the detection limits were found to be as low as 6.75 × 10⁻⁸ M and 7.89 × 10⁻⁷ M, respectively. Furthermore, the practical applicability of this probe has been examined in living cells.

A simple Schiff-base ligand 2-hydroxy-1-naphthaldehyde semicarbazone (HNS) was synthesized and applied to the sequential sensing of Al³⁺ and F⁻ ions in aqueous media and live cells.

A water-soluble molecular probe with aggregation-induced emission for discriminative detection of Al3+ and Pb2+ and imaging in seedling root of Arabidopsis

Luminogens with aggregation-induced emission (AIE) have been used to develop a new type of molecular probes based on analyte-triggered aggregation, but it still remains a challenge to design water-soluble AIE-active probe for specific detection of metal ions. Herein, we designed and synthesized a water-soluble molecular probe with AIE property for discriminative detection of aluminum ion and lead ion. Four carboxylic acid groups were incorporated into a tetraphenylethylene unit to enhance the coordination affinity and increase water-solubility in aqueous solution. The designed probe can be selectively lighted up by aluminum ion and lead ion via coordination-triggered AIE process. Discrimination of aluminum ion and lead ions based on the probe can be achieved in quantitative manner with the assistance of suitable masking reagents. This probe was further used to image aluminum ions in living cells of seedling roots of Arabidopsis, and the results showed that this probe is capable of imaging aluminum ions in living cells avoiding the interference of lead ions, and is suited for long-term imaging due to its excellent photostability. This work expands the application scope of AIE-active probes in discriminative detection of metal ions, and provides a design direction for water-soluble AIE probes to avoid the false signals from self-precipitation under physiological conditions.

Application of a Novel "Turn-on" Fluorescent Material to the Detection of Aluminum Ion in Blood Serum

A novel "turn-on" fluorescent bioprobe, 1,2,3,4,5-penta(4-carboxyphenyl)pyrrole sodium salt (PPPNa), with aggregation-enhanced emission characteristics was synthesized for the in situ quantitative detection of Al³⁺ in serum. It exhibited a high selectivity to Al³⁺ in both simulated serum and fetal calf serum with no interferences from other metal ions or serum components observed and no isolation required. A weak interaction between PPPNa and serum albumin was found, which caused no interference, but enhanced fluorescence response of PPPNa to Al³⁺ and improved detection sensitivity. The limit of detection was determined to be 1.50 μmol/L Al³⁺ in phosphate-buffered saline solution containing 33 μg/mL bovine serum albumin (BSA) and decreased to 0.98 μmol/L as BSA concentration increased to 100 μg/mL. The fluorescence "turn-on" mechanism of the PPPNa probe to detect Al³⁺ was proposed. A bidentate complex is formed between the carboxy group of PPPNa and Al³⁺, causing the photoluminescence (PL) emission enhancement by aggregation. BSA chains further strengthen the stacking compactness of the aggregates of PPPNa and Al³⁺ and consequently enhance the PL emission of PPPNa by further promoting the restriction of intramolecular rotation of the phenyl ring. Its application to the in situ Al³⁺ was successfully demonstrated with HeLa cells and NIH 3T3 cells. The low cytotoxicity and highly selective response of PPPNa to Al³⁺ endow its great potentials to in vivo detecting and imaging of Al³⁺ as well as an absorbent of Al³⁺.

Photophysical properties of a D–π-A Schiff base and its applications in the detection of metal ions

AIEE-active D–π-A Schiff-base H₂L is stimuli-sensitive and acts as a dual channel chemosensor for Cu²⁺ ions and naked-eye probes for Mn²⁺ ions.

Tetraphenylpyrimidine-Based AIEgens: Facile Preparation, Theoretical Investigation and Practical Application

Aggregation-induced emission (AIE) has become a hot research area and tremendous amounts of AIE-active luminogens (AIEgens) have been generated. To further promote the development of AIE, new AIEgens are highly desirable. Herein, new AIEgens based on tetraphenylpyrimidine (TPPM) are rationally designed according to the AIE mechanism of restriction of intramolecular motion, and facilely prepared under mild reaction conditions. The photophysical property of the generated TPPM, TPPM-4M and TPPM-4P are systematically investigated and the results show that they feature the aggregation-enhanced emission (AEE) characteristics. Theoretical study shows the high-frequency bending vibrations in the central pyrimidine ring of TPPM derivatives dominate the nonradiative decay channels. Thanks to the AEE feature, their aggregates can be used to detect explosives with super-amplification quenching effects, and the sensing ability is higher than typical AIE-active tetraphenylethene. It is anticipated that TPPM derivatives could serve as a new type of widely used AIEgen based on their facile preparation and good thermo-, photo- and chemostabilities.

Aggregation induced emission: Concluding Remarks

This article presents the Closing Remarks of the Faraday Discussion on aggregation induced emission (AIE) held in Guangzhou, China in November 2016. The history of the AIE phenomenon is summarized, from its discovery and mechanistic studies to real-life applications in optoelectronics, environmental monitoring and biomedical research. The paper concludes with comments on the future perspectives of the field.

A turn-on fluorescent probe for Cd2+ detection in aqueous environments based on an imine functionalized nanoscale metal–organic framework

A turn-on MOF-based fluorescent probe for Cd²⁺ detection has been developed with high sensitivity and excellent selectivity.

Naphthol-based fluorescent sensors for aluminium ion and application to bioimaging

Three naphthol Schiff base-type fluorescent sensors, 1,3-Bis(2-hydroxy-1-naphthylideneamino)propane (L1), 1,3-Bis(1-naphthylideneamino)-2-hydroxypropane (L2) and 1,3-Bis(2-hydroxy-1-naphthylideneamino)-2-hydroxypropane (L3), have been synthesized. Their recognition abilities for Al(3+) are studied by fluorescence spectra. Coordination with Al(3+) inhibited the CN isomerization of Schiff base which intensely increase the fluorescence of L1-L3. Possessing a suitable space coordination structure, L3 is a best selective probe for Al(3+) over other metal ions in MeOH-HEPES buffer (3/7, V/V, pH=6.6, 25°C, λem=435nm). A turn-on ratio over 140-fold is triggered with the addition of 1.0 equiv. Al(3+) to L3. The binding constant Ka of L3-Al(3+) is found to be 1.01×10(6.5)M(-1) in a 1:1 complex mode. The detection limit for Al(3+) is 0.05μM. Theoretical calculations have also been included in support of the configuration of the L3-Al(3+) complex. Importantly, the probe L3 has been successfully used for fluorescence imaging in colon cancer SW480 cells.

The fluorescent bioprobe with aggregation-induced emission features for monitoring to carbon dioxide generation rate in single living cell and early identification of cancer cells

A novel fluorescent probe, tris (2-(dimethylamino) ethyl)-4,4',4″-(1H-pyrrole-1,2,5-triyl) tribenzoate (TPP-TMAE), with aggregation-enhanced emission (AEE) feature showed a simple, highly selective, specific, and instant response to trace amount carbon dioxide (CO2). Because of this special characteristic, TPP-TMAE is ideal to be a biomarker for in-situ monitoring of the CO2 generation rate during the metabolism of single living cell. The rates in single living HeLa cell, MCF-7 cell, and MEF cell were 6.40 × 10(-6)±6.0 × 10(-8) μg/h, 5.78 × 10(-6)±6.0 × 10(-8) μg/h, and 4.27 × 10(-7)±4.0 × 10(-9) μg/h, respectively. The distinct responses of TPP-TMAE to CO2 generated from cancer cells and normal cells suggested TPP-TMAE as a useful tool for deeper understanding metabolism process and distinguishing cancer cells from normal cells during the early diagnosis of cancers.

Hydrogen bond-assisted aggregation-induced emission and application in the detection of the Zn(ii) ion

The compounds of 3-aminopyridine-2-carboxylic acid with K(+) (1) and Zn(2+) (2) were found to be AIE-active. The AIE behaviours could be attributed to the restriction of intramolecular rotation (RIR) and vibration (RIV) via hydrogen bonds, resulting in rigidity enhancement of the molecules. An AIE-based fluorescence turn on chemosensor for the Zn(ii) ion has been developed in aqueous media with high selectivity and sensitivity.

Biological study of metal–organic frameworks towards human ovarian cancer cell lines

Solvothermal chemistry was employed to produce PCN-95 starting with pyrene forming eight connected Zr6 clusters conforming to an isoreticular structure (P6/mmm). The secondary building unit was determined from polycrystals as having Zr6 center octahedral to and covering all triangular faces with eight OH and carboxylates as the framework. The polycordinated network material was characterized by electron microscopy and evaluated from its cytotoxicity against human ovarian cells. The concentration at 50% inhibition was in the low parts per million and electron micrographs showed a “darker region” around the DNA, indicative of DNA condensation leading to fragmentation. To determine whether the observed biological effects were due to the generation of singlet oxygen or elicitation of specific death pathways, the polycordinated network with and without select inhibitors or reducing and oxidizing agents was employed against human retinal pigment epithelium in vitro and NO measured as a measure of cell health against controls cells (without any cofactors). The NO release was measured a over 3 h time period and the kinetic plots compared and contrasted with known biochemical pathways. The plots were similar to methylene blue, a known singlet oxygen generator and between hydrogen peroxide and citrate but unlike cyanide or rotenone. Collectively, the results support the hypothesis of singlet oxygen generation at the mitochondria and interaction with complex IV of the electron transport chain, facilitating the reduction of nitrite towards more NO. When the cell generates higher concentrations of NO, the apoptotic pathways are triggered resulting in cell death.

A simple nanoporous silica-based dual mode optical sensor for detection of multiple analytes (Fe3+, Al3+ and CN−) in water mimicking XOR logic gate

A simple but versatile nanoporous silica-based optical sensor was synthesized and characterized using different techniques such as XRD, BET, TGA, and FT-IR.

The synthesis of chiral triphenylpyrrole derivatives and their aggregation-induced emission enhancement, aggregation-induced circular dichroism and helical self-assembly

AIEE-active chiral triphenylpyrrole derivatives possess aggregation-induced circular dichroism and circularly polarized luminescence features with self-assembling helical nanofibers.

Quantitation of Albumin in Serum Using "Turn-on" Fluorescent Probe with Aggregation-Enhanced Emission Characteristics

An aggregation-enhanced emission active luminogen named as sodium 4,4'4″-(3,4-diphenyl-1H-pyrrole-1,2,5-triyl)tribenzoate (DP-TPPNa) with propeller construction was synthesized and developed as a "turn on" fluorescent probe for in situ quantitation of albumin in blood serum. The DP-TPPNa fluorescence intensity was linearly correlated with the concentration of two serum albumins, bovine serum albumin (BSA) and human serum albumin (HSA), in pure PBS buffer in the ranges of 2.18-70 and 1.68-100 μg/mL, respectively. The detection limits were as low as 2.18 μg/mL for BSA and 1.68 μg/mL for HSA. The response time of fluorescence to serum albumin (SA) was very short (below 6 s), which achieved real-time detection. It also showed high selectivity to SA because other components in serum barely interfere with the detection of DP-TPPNa to SA, enabling in situ quantitative detection of SA without isolation from serum. DP-TPPNa was successfully applied for the quantitative detection of BSA in fetal bovine serum. The mechanism of fluorescent turn-on behavior was elucidated utilizing an unfolding process induced by guanidine hydrochloride, which revealed a capture process via selective hydrophobic interaction and hydrogen bonding between luminogen and SA.

A New Fluorescence Turn-On Probe for Aluminum(III) with High Selectivity and Sensitivity, and its Application to Bioimaging

The development of novel selective probes with high sensitivity for the detection of Al(3+) is widely considered an important research goal due to the importance of such probes in medicine, living systems and the environment. Here, we describe a new fluorescent probe, N'-(4-diethylamino-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (1), for Al(3+). Probe 1 was evaluated in a solution of acetonitrile/water (1:1 v/v). Compared with previously reported probes for Al(3+), probe 1 can be synthesized easily and in high yield. A Job plot confirmed that probe 1 is able to complex Al(3+) in a 1:1 ratio, and the binding constant was determined to be 4.25×10(8) m(-1). Moreover, the detection limit was as low as 6.7×10(-9) m, suggesting that probe 1 has a high sensitivity. Common coexistent metal ions, such as K(+), Co(2+), Ca(2+), Ba(2+), Ni(2+), Pb(2+), Hg(2+), Ce(2+), Zn(2+), Cd(2+), Fe(3+), showed little or no interference in the detection of Al(3+) in solution, demonstrating the high selectivity of the probe. Finally, the ability of probe 1 to act as a fluorescent probe for Al(3+) in living systems was evaluated in Gram-negative bacteria, Escherichia coli, and confocal laser scanning microscopy confirmed its utility. The results of this study suggest that 1 has appropriate properties to be developed for application as a fluorescent probe of Al(3+) for use in biological studies.

Nanomolar fluorogenic detection of Al(III) by a series of Schiff bases in an aqueous system and their application in cell imaging

Three positional isomers of a Schiff base containing -OH as end groups have been synthesized and evaluated for selective Al(III) detection due to inhibition of ESIPT, PET and activation of CHEF in 70% aqueous medium. Devoid of any conventional fluorophore, these sensors have nanomolar detection limits with high quantum yields and naked eye sensing of Al(III). Moreover, these probes have been demonstrated to enable the Al(III) detection in live human HeLa cells and rat C6 glioma cells using a confocal microscope.